The liquid crystal display of an active matrix type includes numerous thin film transistors (TFTs) that can control the display of pixels constituting a flat display panel. The organic EL display includes organic light emitting diodes (OLED), i.e., organic EL elements, and can be arranged as a flat display panel of an active matrix type.
FIG. 20 shows an example of a circuit arrangement of one pixel (i.e., a pixel circuit) in an active organic EL display apparatus. A gate line (Gate) extending in the horizontal direction can supply a high-level gate signal to a selection TFT 2. A data line (Data) extending in the vertical direction can supply a data signal having a voltage level corresponding to required display brightness. When the data signal is applied to the data line under the condition that the selection TFT 2 is turned on, a holding capacitor C can store the data signal. A driver TFT 1 can produce driving current corresponding to the data signal which is supplied to an organic EL element 3. The organic EL element 3 can emit light corresponding to the data signal.
The light emission of an organic EL element is substantially proportional to its current. In general, a predetermined voltage (Vth) is applied between the gate terminal of the driver TFT 1 and a PVdd terminal, so that the drain current starts flowing in the vicinity of the black level of an image. Furthermore, the amplitude of an image signal can be determined, so that predetermined brightness can be obtained in the vicinity of the white level.
FIG. 21 is a graph showing the prior art relationship between an input signal voltage of the driver TFT 1 (i.e., gate-source voltage Vgs=a difference between the voltage of data line “Data” and a power source voltage PVdd) and a current icv flowing in the organic EL element 3 (i.e., current corresponding to brightness). The gradation of the organic EL element 3 is adjustable with an appropriately determined data signal, so that the voltage Vth can define a black level voltage and the voltage Vw can define a white level voltage.
The organic EL display apparatus can be configured into a display panel including numerous pixels disposed in a matrix pattern. Such a display panel tends to be prone to manufacturing errors or deterioration with age, and the threshold voltage (Vth) of a driver TFT or the gradient (gm) of voltage-current (V-I) characteristics may change undesirably. As a result, the pixels constituting a display panel will cause brightness irregularities.
To correct the brightness irregularities, as shown in FIG. 1, the threshold voltage (Vth) can be corrected by adding an appropriate value to the driving signal of each pixel (referred to as “offset correction”), or the gradient (gm) can be corrected by multiplying by an appropriate value (referred to as “gain correction”).
For example, an arrangement shown in FIG. 2 can be used to correct the brightness irregularities. When the average characteristic of pixels aligned in a horizontal line n (refer to curve (b)) is different from the average characteristic of all pixels (refer to curve (a)), brightness irregularities can be corrected by changing offset/gain of the horizontal line n.
Similar correction methods are, for example, disclosed in Japanese Patent Application Laid-open No. Hei 11-282420, U.S. Patent Application Publication 2004/0150592 A1, and WO 2005/101360 A1.
If the above-described correction is required for all pixels, correction data must be prepared for all pixels correspondingly. In other words, a large capacity of memory will be required to store correction data necessary for the pixels constituting the panel. The capacity and cost of a required memory will increase in accordance with the number of pixels constituting the panel. The required memory size will further increase if enlarging the bit width of a correction memory is required to correct a wide range of irregularities, correspondingly.